1. Field of Invention
This invention relates to a control system for an automotive transmission.
2. Description of the Related Art
An automatic transmission for use in an automobile comprises a transmission disposed between an engine and driving wheels of the car, and a select lever provided in a driver's compartment, and is applied to control changing gear operation of the transmission according to the position of the select lever.
The select lever can be shifted between a neutral position (P (Parking) range, N (Neutral) range) and a running position (D (Drive) range, R (Reverse) range, etc.). When the select lever is shifted to the neutral position, the transmission becomes in a neutral state where power output of the engine is not transmitted to the driving wheels. When the select lever is shifted to the running position, the transmission becomes in a gear-in state (or an engaging state) where driving force of the engine is transmitted to the driving wheels. In the gear-in state, with a so-called creep of the transmission, the vehicle is moved forward when the select lever is in D range, or is moved backward when the select lever is in R range, even if an accelerator is not depressed.
Such type of automatic transmission is known that the select lever cannot be shifted from the neutral position to the running position unless a brake pedal is depressed. Thereby, (1) the driver develops a habit of shifting the select lever after depressing the brake pedal so that mistake in depressing the brake pedal or the accelerator upon sudden movement of the vehicle does not happen, and (2) sudden movement of the vehicle when the select lever is touched by the human body in error and shifted from the neutral position to the running position during the vehicle stopping can be prevented.
Many of these types of automatic transmissions have a mechanism for mechanically locking the select lever into the neutral position to prevent the select lever to be shifted from the neutral position to the running position without depression of the brake pedal. Alternatively, so-called shift-by-wire type automatic transmission is known in which the creep can be generated. In this, the select lever can be shifted from the neutral position to the running position even if the brake pedal is not depressed; however, the gear-in operation of the transmission is not done by an electrical control restriction (refer to Japanese Patent Publication No. 4-77825).
Meanwhile, the conventional automatic transmission cannot change its state from the neutral state to the gear-in state unless the brake pedal is depressed. Therefore, when the vehicle sticks into a hollow on the road with a low coefficient of friction such as a sand road, a gravel road or a mud road, it is very difficult to perform a shaking operation by which the vehicle is shaken forward and backward to escape from the hollow.
More specifically, in order to escape from the sticking state, it is effective to suddenly generate a driving force that oppositely alternates its direction to change a moving direction of the vehicle from forward to backward and vice versa by turns to shake the vehicle forward and backward. However, in order to change the moving direction of the vehicle, it is required to change by turns the position of the select lever from the N range as the neutral position to the D range or the R range as the running position. In the conventional automatic transmission, the brake pedal must be depressed whenever the select lever is shifted from the N range to the D range or the R range which are located in both sides of the N range. Therefore it has been difficult to perform a quick change of the direction of the driving force.